This invention relates to a controlled-impedance coaxial cable interconnect system.
Referring to FIG. 1, a conventional semiconductor integrated circuit tester includes multiple tester channels 2 each having an I/O terminal for connection through a signal path 6 to a terminal of an integrated circuit device under test (DUT) 10 for supplying a stimulus signal to, or receiving a response signal from, the DUT.
It is necessary that the signal paths 6 should have sufficient bandwidth to propagate the test signals (stimulus and response signals) between the DUT terminals and the I/O terminals of the tester channels without undue degradation. Accordingly, it is conventional to implement the signal paths 6 with transmission line structures. As the frequency of operation of integrated circuits increases, the frequencies of the test signals that are utilized in evaluating an integrated circuit device increase and accordingly the bandwidth of the signal paths 6 must increase. It is well known that a transmission line structure of which the characteristic impedance is uniform throughout its length will have a higher bandwidth than a transmission line structure of which the characteristic impedance varies significantly over its length.
FIG. 2 shows a portion of the conventional tester in more detailed, but still highly schematic, form. FIG. 2 illustrates a device interface board (DIB) 14 that is provided on one surface with a socket (not shown) for receiving the DUT in packaged form and having multiple contact pads 18 exposed at its other surface and connected through conductive traces of the DIB to respective terminals of the socket. The DIB is illustrated in FIG. 2 in a horizontal orientation with the contact pads 18 on the lower surface of the DIB. In this case, the DUT socket would be on the upper surface of the DIB. It will be appreciated by those skilled in the art that this choice of orientation of the DIB 14 is for the sake of convenience and that other orientations may be employed. It will also be appreciated that although, for convenience, only one socket has been referred to, for receiving a single integrated circuit device for testing, it is common to provide multiple sockets on a single DIB for concurrent testing of multiple integrated circuit devices.
The pads 18 are arranged in several discrete groups on the lower surface of the DIB and the tester includes a positioning block 22 for each group of pads 18. The positioning blocks may be mounted in a carrier that is restrained against horizontal movement relative to the DIB and is displaceable vertically relative to the DIB by a force mechanism (not shown). The positioning block 22 is formed with multiple apertures 26 aligned with the pads 18 respectively.
The portion of the signal path between a pad 18 of the DIB and the I/O terminal of the corresponding tester channel 2 is implemented by a coaxial transmission line structure. The coaxial transmission line structure includes a coaxial cable 28 that is composed of an inner conductor 30, an outer shield conductor 32 spaced from the inner conductor, dielectric material 34 between the inner conductor and the outer conductor, and an insulating jacket 38. The coaxial cable 28 is connected at one end to the terminal of the tester channel 2 and is provided at its opposite end with a pogo pin connector 42 that includes a cylindrical metal fitting 46 in electrically conductive contact with the outer conductor 32 of the cable, a dielectric sleeve 50 in the metal fitting, and a spring probe pin 54, commonly referred to as a pogo pin, mounted in an axial bore in the dielectric sleeve. The conventional pogo pin, which is shown in simplified form in FIG. 2, includes a metal barrel 56 having an internal stop 58, a plunger 60 that is a sliding fit inside the barrel and is captive within the barrel, and a spring 62 effective between the internal stop and the plunger for urging the plunger toward a projecting position. The inner conductor 30 of the cable 28 is fitted in the open end of the metal barrel 56, and the barrel 56 and the spring 62′ provide an electrically conductive connection between the inner conductor 30 and the plunger 60.
The pogo pin connector 42 is secured in an aperture 26 in the positioning block. As shown in FIG. 2, the tip of the plunger 60 projects above the upper surface of the positioning block. When the carrier is displaced upwards by the force mechanism, the tip of the plunger engages the contact pad 18 and accordingly the pogo pin provides an electrically conductive connection between the inner conductor 30 and the contact pad 18. Ground connections are provided between the outer conductor 32 and the ground traces of the DIB by pogo pins 68 that are secured directly in the positioning block, without interposition of dielectric material, and engage contact pads 70 that are connected to the ground traces of the DIB.
The characteristic impedance of a coaxial transmission line is a function of the dielectric constant of the dielectric material and the ratio of the external diameter of the inner conductor to the internal diameter of the outer conductor.
Conventionally, the DIB is manufactured so that the segments of the signal path within the DIB are of uniform 50 ohm characteristic impedance and the coaxial cables that connect the I/O terminals of the tester channels to the pogo pin connectors 42 are of uniform 50 ohm characteristic impedance. The portion of the interconnect system shown in FIG. 2 between the DUT end of the coaxial cable and the contact pad 18 of the DIB is designed to approximate a coaxial transmission line having a uniform characteristic impedance of 50 ohms.
The interconnect system shown in FIG. 2 is satisfactory for many purposes, but it can be seen that there is potential for discontinuities in characteristic impedance of the signal path due to variations in geometry of the conductors and variations in dielectric constant of the dielectric material between the conductors of the coaxial transmission line structure.
As integrated circuits have increased in complexity, the number of terminals of IC devices has increased and in order to avoid increasing the size of the DIB to accommodate additional contact pads, it has become desirable to pack the signal pads 18 more densely on the DIB. This in turn necessitates that the pogo pin connectors 42 be packed more densely in the positioning block. In a practical implementation of the interconnect system that is shown in FIG. 2, the positioning block 22 is provided with apertures for receiving sixteen pogo pin connectors 42 for accessing an array of sixteen signal pads 18, as shown in FIG. 3. The density with which the connectors 42 can be packed is limited by the physical dimensions of the connector 42 and coaxial cable 28, which are relatively large in diameter in part because the dielectric constant of the dielectric material necessitates that the outer conductor be of substantially greater diameter than the inner conductor in order to provide the desired 50 ohm characteristic impedance.
Coaxial cable in which air is the principal dielectric material between the inner and outer conductors is commercially available. Because the dielectric constant of air is much lower than that of the synthetic dielectrics (such as PTFE) that have hitherto been commonly used in coaxial cables, in an air dielectric cable the ratio of the internal diameter of the outer conductor to the diameter of the inner conductor can be substantially less than in a coaxial cable that employs a synthetic dielectric as the principal dielectric material and accordingly for a given diameter of the inner conductor, the thickness of the cable can be substantially less.
One type of air dielectric coaxial cable is known as air dielectric microfilament coaxial cable. Air dielectric microfilament coaxial cable typically comprises an inner conductor, a thin-walled tube of PTFE inside the outer conductor and of internal diameter greater than the external diameter of the inner conductor, a coil of fine PTFE filament material wound around the inner conductor in the space between the inner conductor and the PTFE tube to maintain a uniform spacing between the inner and outer conductors, and a protective jacket of insulating material.
Several interconnect technologies have been developed for providing electrical contact between closely spaced pins of an integrated circuit device and a corresponding array of conductive lands on a dielectric substrate. Such technologies include the ball grid array and the land grid array. A typical land grid array comprises a precision molded retaining member made of dielectric material and formed with apertures distributed in a rectangular array corresponding to the array of conductive lands on the dielectric substrate. Each aperture contains a spring contact. When the contact device is clamped between the integrated circuit device and the dielectric substrate, the spring contact elements enter electrically conductive pressure contact with the conductive lands on the substrate and the corresponding pins of the integrated circuit device. A land grid array that employs C-shaped spring contacts is commercially available under the designation InterCon cLGA.